Add client and host dispatchers for video_stats channel to send video stats from host to client.

The new channel will be used with WebRTC protocol to send video stats to
client to make it possible to measure round-trip latency.

BUG=621691

Review-Url: https://codereview.chromium.org/2176443002
Cr-Commit-Position: refs/heads/master@{#407221}
15 files changed